When it comes to shaping the intensity patterns, wavefronts of light, or position of an image plane or focus, fixed lenses are convenient, but often the need for frequent reshaping requires adaptive optical elements. Nonetheless, people typically settle for whatever comes out of their laser, be it Gaussian or top hat, and use fixed lenses to produce a beam with the desired characteristics. In laser micromachining, for instance, a microscope objective will provide a sharply focused region of given area that provides sufficient power density to ablate the materials.
However, in a variety of applications, it is useful or even necessary to have feedback between the beam properties of the incident light and the materials processes that are induced. A classical example is using a telescope to image distant objects through the atmosphere. In this case, the motion of the atmosphere causes constant perturbations in the wavefront of the light. One can measure the fluctuations and, using adaptive elements, adjust the wavefront to cancel out these effects. Still other, laboratory-based, imaging applications such as ophthalmologic scanning, confocal microscopy or multiphoton microscopy on living cells or tissue, would benefit greatly from the use of direct feedback to correct for wavefront aberrations induced by the sample under investigation, or to provide rapid scanning through focal planes.
Advanced materials processing applications also require precise beam intensity or wavefront profiles. In these cases, unlike imaging, one is modifying the properties of a material using the laser. For instance, laser forward-transfer techniques such as direct-write printing can deposit complex patterns of materials —such as metal oxides for energy storage or even living cells for tissue engineering—onto substrates. In this technique, a focused laser irradiates and propels a droplet composed of a mixture of a liquid and the material of interest toward a nearby substrate. The shape of the intensity profile of the incident laser plays a critical role in determining the properties of the deposited materials or the health of a transferred cell. In cases such as these, the ability to modify the shape of the incident beam is important, and with the ability to rapidly change the shape, one adds increased functionality by varying the laser-induced changes in a material in a from one spot to another.
Even traditional laser processes like welding or cutting can benefit from adaptive optical elements. In welding, a continuous-wave laser moves over a surface to create a weld bead between the two materials. Industrial reliability requires uniform weld beads, but slight fluctuations in the laser, the material, or the thermal profile can diminish uniformity. Therefore, with feedback to an adaptive optical element, more consistent and regular features are possible.
Whether the purpose is to process material, or simply to create an image, the applications for adaptive optics are quite varied. Some require continuous-wave light, others need pulsed light, but the unifying requirement of all applications is to have detailed control over the properties of the light, and to be able to change those properties rapidly so that the overall process can be optimized.
Fixed optical elements give great choice in selecting the wavefront properties of a beam of light, but there exist few techniques for modifying the beam temporally. The simplest approach is to mount a lens or a series of lenses on motion control stages. Then one can physically translate the elements to deflect or defocus the beam. For instance, this technique is useful for changing the focus of a beam in order to maintain imaging over a rough surface, or changing the spot size of a focused beam on a surface for laser micromachining. However, this approach suffers from a drawbacks related to large scale motion such as vibrations, repeatability and resolution. Moreover, it can be slow and inconvenient for many industrial applications where high reliability and speed are needed. Nonetheless, for certain applications such as zoom lenses on security cameras, this is a satisfactory technique. Recently, more advanced methods of inducing mechanical changes to lenses involve electric fields or pressure gradients on fluids and liquid crystals to slowly vary the shape of an element, thereby affecting its focal length.
When most people think of adaptive optical elements, they think of two categories, digital micromirror arrays and spatial light modulators. A digital mirror array is an array of small moveable mirrors that can be individually addressed, usually fabricated with conventional MEMS techniques. The category also includes large, single-surface mirrors whose surfaces can be modified with an array of actuators beneath the surface. In either case, by controlling the angle of the reflecting surfaces, these devices modulate the wavefront and shape of light reflected from them. Originally digital mirror arrays had only two positions for each mirror, but newer designs deliver a range of motion and angles.
Spatial light modulators also modify the wavefront of light incident on them, but they typically rely on an addressable array of liquid crystal material whose transmission or phase shift varies with electric field on each pixel.
Both digital mirror arrays and spatial light modulators have broad capabilities for modulating a beam of light and thereby providing adaptive optical control. These are digitally technologies and can therefore faithfully reproduce arbitrary computer generated patterns, subject only to the pixilation limitations.
These devices have gained widespread use in many commercial imaging and projecting technologies. For instance, digital mirror arrays are commonly used in astronomical applications, and spatial light modulators have made a great impact on projection television and other display technology. On the research front, these devices have enabled a myriad of new experiments relying on a shaped or changeable spatial pattern such as in optical manipulation, or holography.
Although current adaptive optical technologies have been successful in many applications, they suffer from limitations that prevent their use under more extreme conditions. For instance, one of the major limitations of spatial light modulators is the slow switching speed, typically on the order of only 50-100 Hz. Digital mirror arrays can be faster, but their cost can be prohibitive. Also, while these devices are good for small scale applications, larger scale devices require either larger pixels, leading to pixilation errors, or they require an untenable number of pixels to cover the area, decreasing the overall speed and significantly increasing the cost. Finally, these devices tend to have relatively low damage thresholds, making them suitable for imaging applications, but less suitable for high energy/high power laser processing. Accordingly, there is a need the in the field of adaptive optics for devices which can overcome current device limitations, such as speed and energy throughput for materials processing applications.